This application is in response to the pilot research topic number 25 (Vaccines and immune response). The principal investigator is a new investigator. This project will examine the relationship between serum and colonic antibody responses to Clostridium difficile and disease expression. It is a preliminary clinical study designed to provide essential information for the development and testing of a vaccine against C. difficile for use in elderly populations. C. difficile-associated disease (CDAD) is an important iatrogenic, nosocomial condition which affects predominantly older patients. Conventional antibiotic therapy of this condition is associated with a high relapse rate and the emergence of antimicrobial resistance among micro-organisms. Immunotherapy with antibody products is being investigated as an alternative form of therapy and work is underway to develop a human vaccine for the prevention of CDAD. Knowledge of the kinetics of the immune response, particularly in relationship to aging, and the antigen specificity and level of antibody needed to protect against disease would be vital for vaccine development. Our preliminary studies suggest an association between serum and colonic antibody response and disease expression following colonization. However, interpretation of these preliminary studies is limited by their small sample sizes, retrospective study design, and failure to control for potential confounders, such as age. We will identify and prospectively follow a cohort of 300 hospitalized patients with risk factors for C. difficile. Serial stool and serum samples will be obtained to identify patients colonized with C. difficile and to examine the kinetics of the immune response to colonization. Serum IgG, IgM and IgA, fecal IgA and IgG against C. difficile toxins and non-toxin antigens will be measured by ELISA. Anti-toxin neutralizing activity will be assessed by tissue culture cytotoxicity assay. Adult patients of all ages will be studied, to determine the effect of age on antibody response. Patients with different outcomes will be identified (no colonization, asymptomatic colonization, symptomatic colonization, severe disease, relapsing disease) and compared in terms of their serum and colonic anti-C. difficile antibody responses. By this process, we hope to determine the antigen-specific antibody and level, if any, which protects patients against colonization, disease expression, severe and relapsing disease. These data will be used to establish surrogate markers of protection for use in upcoming human C. difficile vaccine trials.